THE WORK OF THE OCEAN. 335 



the general movements of the atmosphere; (2) periodic movements 

 which are not primarily circulatory, brought about by the attraction 

 of the sun and moon; and (3) aperiodic movements, due to occasional 

 catLses, stich as eaithciuakes, volcanic explosions, landslides, etc., which 

 determine local and temporary movements, often of exceptional strength. 



Differences in density and their results. — Differences in density result 

 from chfferences in temperature and sahnity. Temperature alone 

 considered, water wotild be densest where it is coldest, namely in the 

 polar regions. Differences in sahnity result from differences in evapo- 

 ration and from inequalities in the supply of fresh water. Evaporation 

 alone considered, the sea-water should be densest where evaporation is 

 greatest; but the ecjuatorial region, where evaporauon is greatest, is 

 also a region where precipitation is heavy, and precipitation, by freshen- 

 ing the water, opposes the effect of great evaporation. The greatest 

 differences in density due to the tmequal supply of fresh water are to 

 be fotmd near the borders of continents, where the precipitation on 

 the land is discharged into the sea. In the polar regions, the great 

 supply of fresh water, especially during the season when the ice is melt- 

 ing, opposes the effect of the low temperature, so far as the density of 

 the water is concerned. The result of the operation of these factors 

 affecting the density of the sea-water is to insme a general circulation, 

 chrected to the end of ec^uahzing the densities; and since the disttubing 

 factors are constantly in operation, equilibrimn is never established, 

 and the movements of the water are perpetual. 



The presstue gradients resulting from differences of density are so 

 shght that the resulting movements are scarcely more than a creep of 

 the waters. In general they are far too slow to be of importance in 

 gradational work; but the earth's rotation deflects the creeping waters 

 and tends to concentrate the ec|uator-ward movement into currents 

 on the east sides of the continents, and the pole-ward movement on 

 the west sides. In favorable situations these currents may be com- 

 petent to produce sensible mechanical results. Even where tliis is not 

 the case the circulation helps to equahze the temperatures of the sea, 

 and so of the air above and of the land about. Indirectly, therefore, 

 the circula ion of the ocean- waters affects every geological process 

 wliich is sensitive to climate. 



Differences in level and their results. — While the surface of the 

 ocean is the common datum plane to wlhch elevations and depressions 



